1. Field of the Invention
This invention relates to a battery saving apparatus and method for supplying power to a selective call communication receiver for enabling the detection of a synchronization codeword in a predetermined selective call signaling format, and more particularly, for supplying power to the selective call communication receiver for enabling an optimal detection of the synchronization codeword following either a variable bit length preamble or data transmitted in a different signaling format, so as to provide an improved battery saver function.
2. Description of the Prior Art
The are numerous selective call signaling formats in use today which include a synchronization codeword for providing a selective call receiver, such as a pager, the ability to be synchronized with transmitted selective call messages. One such system which has been widely used is the POCSAG signaling format, shown in FIG. 1. The POCSAG signaling format includes a preamble (P) which is a pattern of bit reversals, 0101010. . . , transmitted at an address and data bit rate, or baud rate, of 512 or 1200 bits per second. The preamble is followed by one or more batches (B) of selective call messages, each batch commencing with the synchronization codeword (SC) followed by eight frames (F) containing two codewords. Each codeword is a thirty-two bit binary word, representing the address or message information. Pagers operating in a POCSAG system are assigned to one of the eight frames to provide a battery saving function during the transmission of the addresses and messages. The preamble is transmitted for at least 576 bit times, which corresponds to the time required to transmit at least one complete batch of selective call messages plus one additional codeword. This, in principle, insures that a sufficient number of preamble bits are transmitted for the selective call receiver to attain bit synchronization, and for the selective call receiver to receive at least one complete synchronization codeword to attain word synchronization.
In normal operation, when a prior art pager in a POCSAG system had not detected the synchronization codeword on the channel during two consecutive batches, the normal battery saver cadence was interrupted and power was periodically supplied to the receiver in less frequent time intervals, allowing any data on the channel to be monitored and detected as the pager searched for the synchronization codeword. When data was detected on the channel during the periodic supply of power, power to the receiver was maintained for an additional time interval to allow the detection of the synchronization codeword. When the synchronization codeword was not detected during the balance of the time interval, power to the receiver was terminated until the next periodic time interval. Failure to detect the synchronization codeword during the extended time interval was often the result of the pager falsing on noise, or falsing on data transmitted in a different signaling format which shares the use of the same channel. The occurrence of false data detections on shared channels, especially where the signaling formats sharing the channel have the same or substantially the same baud rates, can be significant, being as much as a ten per cent false detection rate and more, depending on the nature of the data on the channel. As a result, prior art pagers have generally been designed to operate at, or close to, the minimum 576 bit time repetition rate, required to optimally detect the synchronization codeword. Battery life was maximized in this instance, even with the false detection of data on the channel, however, at the expense of possibly missing a page, as previously described.
While the battery life was optimized by operating the pager at a 576 bit time repetition rate, other problems were encountered. When a transmitter is keyed, the transmitter is not instantaneously up to full power. Consequently, transmission of the preamble has to be delayed, to guarantee the complete transmission of the preamble. As a result, a transmitter warm-up leader was often provided prior to the transmission of the preamble. Such a warm-up leader had to be selected so as to not generate false data detection, otherwise power could be supplied to the receiver during the warm-up leader time, which would often result in missing the detection of the synchronization codeword. This in turn would result in missing a message directed to the pager. One prior art pager solved this problem by generating a synchronization codeword search window approximately twice as long as the optimum 576 bit time repetition rate when data was detected on the channel. While this guaranteed the detection of the synchronization codeword, it was at the expense of battery life which was reduced by false data detections on the transmitter warm-up leader, on noise, and on data in different signaling formats which shared the same channel.
A method for supplying power to a selective call communication receiver at the optimum batch interval is required to maximize receiver battery life. Such a method, must however, also allow for the detection of the synchronization codeword during intervals of time which can be significantly greater than the optimum 576 bit time batch time interval.